While effective in treating some neurological disorders, neural prostheses are limited because they can excite neurons but not efficiently inhibit them. Direct current (DC) applied to a metal electrode in contact with neural tissue can excite or inhibit neural activity; however, DC stimulation is biologically unsafe because it causes electrochemical reactions at the metal electrode-tissue interface. To avoid these safety hazards, neural prostheses generally deliver alternating current (AC) pulses to evoke action potentials.
While cochlear and retinal prostheses use AC pulses to encode sensory information by modulating firing rate of the afferent fibers above their spontaneous activity, other neural prosthesis applications have substantial difficulties achieving effective treatment with excitation alone. A prosthesis to assist micturition, for instance, requires both excitation of sacral nerves to activate the detrusor muscle and simultaneous inhibition of lumbar nerves to relax the urethral sphincter. For proper balance as well, inner ear vestibular afferent fibers require not only excitation to encode head motion toward the stimulated side of the head, but also inhibition to encode head motion away from it. In restoring normal physiology, therefore, the ability for a neural prosthesis to both inhibit and excite neurons would be useful. Furthermore, several disorders characterized by high neural firing rates such as tinnitus, chronic pain, and epilepsy could be effectively treated by prostheses capable of neural inhibition. Gradual modulation of extracellular potential rather than evoking or inhibiting spikes could further extend the capabilities of neural prostheses to treating disorders such as autism by addressing excitatory vs. inhibitory imbalance, and DC potential support to treat strial hearing loss.
At low amplitudes, DC can achieve graded control of neural activity by altering the extracellular electric field near the electrode. By altering the electric field, DC modulates neural firing thresholds, increasing or decreasing the likelihood of spike initiation. At higher DC amplitudes, cathodic current excites neurons, while anodic current inhibits them. DC stimulation that does not produce electrochemical reactions at the electrode-saline interface could enable more versatile treatments of neurological disorders than what is currently possible.
A solution to the problem of DC stimulation safety is to direct the DC flow of ions into the target tissue by switching mechanical valves in phase with AC square waves applied to the electrodes, which are immersed in an ionic solution. This approach removes DC from the electrode-saline interface, while maintaining DC ionic current through the tissue. FIG. 1A shows the two potential states of the system. FIG. 1B shows the output of the fully-functioning SDCS as the valves operate in synchrony with AC delivered to the electrodes. One important aspect of this device is that it works in a bipolar configuration so that the ions flowing into one tube are replenished by the same types of ions flowing out of the other tube thus resulting in net zero ionic change in the tissue between the two tubes. This configuration addresses pH changes that could potentially be harmful to the neural tissue.
The fidelity of the DC signal is degraded by periodic interruptions in current flow due to non-ideal behavior of the mechanical valves used in the device, as illustrated in FIG. 1B. The interruptions occur because ionic current bypasses the tissue when the valves are temporarily and simultaneously either open or closed during open-to-close and close-to-open transitions. For example, if B2 and A2 are both closed during a transition, no current will flow through the tissue. The duration of the interruptions depends on the speed of the valve transitions from open-to-close and from close-to-open states. Any interruption in the DC current flow however will cause the undesirable volley of neural activity in the target neurons.
It would therefore be advantageous to provide a method to remove the interruptions in the ionic direct current flow.